In general, undesired or non-ideal characteristics, such as transmission impairments (e.g. DC offset and in-phase/quadrature-phase (IQ) imbalance), degrade performance of mobile transceivers. The DC offset is the effect of self mixing by a mixer, and occurs when a signal of a local oscillator (LO) returns after leaking toward an antenna or when a radio frequency (RF) modulation signal input through the antenna is leaked to the local oscillator. Another way to create DC offset is through an inherent offset in the amplifiers due to imbalances. If the DC offset is amplified by amplifiers in the signal path, then this way may saturate a baseband circuit.
The IQ imbalance is caused when the phase difference between the in-phase (I) channel signal and the quadrature-phase (Q) channel signal generated in an oscillator of a wireless transmitter is not 90 degrees. The IQ imbalance can be reduced by designing mixers of the I channel demodulator and the Q channel demodulator to be precisely 90 degrees in phase delay (i.e., orthogonal) to each other. However, designing the mixers so that there is precisely a 90 degrees phase difference to each other is not practical over process and temperature variations. This is because in the layout, the I and Q paths to the mixers traverse different lengths despite the best effort of keeping everything symmetrical. This is especially true for multi-band systems. An IQ imbalance increases the Bit Error Rate (BER), thereby degrading the performance of the wireless transceiver. Moreover, IQ imbalances results in distorted images of the wanted signal being created. These distorted images may superimpose over the wanted signal, thereby degrading performance of the wanted signal. Current methods of dealing with IQ imbalances seek to both reduce the IQ imbalance and filter out the distorted images of the wanted signal.
Such current methods to correct IQ imbalances are determined by the dynamic range of the receiver used to detect the IQ imbalance. If the signal of interest and its image can not be observed with the available dynamic range, the image can not be detected or seen by processing circuitry that attempts to compute the complex image power. For example, if the image of 60 dB below the signal of interest, an analog to digital converter (ADC) with a 60 dB dynamic range will not show the image signal, particularly when the gain of the receive path is reduced. Hence, ADCs with high dynamic ranges are helpful for detecting the image signal when a reasonable headroom is desired between the signal received and the maximum input level of the ADC.
This issue becomes worse when higher image rejection is desired to allow a higher separation between the frequency of a local oscillator and the carrier frequencies. Hence, the dynamic range desired directly depends on the frequency separation and the amount of resulting desired resistance to VCO pulling in the design.
Therefore, there is still a need to improve such IQ imbalance compensation, and a need to reduce transmission impairments and the associated distortion in mobile transmitters.